Talk to the fisherman by the dewford gym and when he askes you if you have an itch to fish say yes,or do what RaikouTGC sayd and if you've beaten norman teach its to Zigzagoon, Linoone, Makuhita, or Hariyama.

Go to dewford town. Talk to the guy standing right side of the Gym.TALK TO HIM AND SAY YES and you will get the ' OLD ROD' which can be used from key item in bag. You will encounter magikarp , Tentacool & Feebas using this rod

If you are in Petalburg and have beaten your dad, either get the old man to take you to dewford (if he is there) or make your way back through Whismur cave from Rustboro (you need Rock Smash). From there go to slateport and get the old man to take you to dewford. in dewford the "fishing guru" is outside the gym and he will give it to you. however, Zigzagoon, Makuhita and their evolutions can also learn surf so you can catch one of those near littleroot town if absolutely necessary.